This invention relates to systems for analyzing cardiac data, and more particularly to systems for analyzing specific portions of cardiac waveforms to determine how to optimize the programming of cardiac stimulating devices to best serve a patient's needs.
Implantable cardiac stimulating devices, such as pacemakers, are well known. Fairly sophisticated devices are presently available that apply electrical pulses to a patient's heart in response to various measured cardiac events. Because each patient has a different cardiac condition, many cardiac stimulating devices are provided with programmable settings which govern their operation. A physician can tailor a cardiac stimulating device to a patient's individual needs by adjusting these settings using a "programmer," as described in commonly-assigned U.S. Pat. No. 4,809,697 to Causey, III et al.
Some of the programmable settings are detection criteria, which the cardiac stimulating device uses to identify cardiac events. For example, a cardiac stimulating device that measures a patient's internal heartbeat signal--known as the ventricular intracardiac electrogram (IEGM)--may determine which regions of the IEGM signal correspond to R-waves based on a comparison of the measured signal to several programmable detection criteria, including a predetermined amplitude threshold. Unless an R-wave has an amplitude greater than the threshold, it will not be detected by the cardiac stimulating device.
In order to ensure that the detection criteria are set properly, a physician can simultaneously monitor the operation of a patient's cardiac stimulating device and the patient's IEGM. The measured IEGM is generally transmitted to a programmer by the cardiac stimulating device via telemetry. The programmer can be used to display both the IEGM data and various other information such as a patient's electrocardiogram (EKG) on a display monitor or chart recorder. Event markers, such as the letter "R" to represent a detected R-wave, can also be displayed adjacent to the region of the IEGM signal that corresponds to the event. Markers are generated by the cardiac stimulating device whenever a cardiac event is detected.
Analysis of the patient's cardiac rhythm using markers can be used to establish if other operating parameters of the cardiac stimulating device are set optimally. Markers are generated when cardiac events are detected by the cardiac stimulating device, such as P-waves for atrial depolarization and R-waves for ventricular depolarization. In addition, the cardiac stimulating device may generate markers to signal significant timing events, such as the end of a refractory period or minimum tracking interval. If desired, the markers can be used in conjunction with IEGM and EKG signals, for example, to establish if the refractory period should be shorter, to sense a faster atrial rate, or longer, to avoid retrograde atrial events.
An apparatus for displaying event markers adjacent to an IEGM signal is described in commonly-assigned U.S. Pat. No. 4,791,936 to Snell et: al. Such an apparatus can be useful in aiding a physician to determine whether the various cardiac stimulating device settings are adjusted properly. For example, the physician can examine the IEGM and EKG signals and make a judgement as to whether the event markers displayed adjacent to various cardiac events appear to be appropriate and whether the operation of the device seems to be correct.
Nevertheless, even if the physician suspects that the cardiac stimulating device is not operating optimally, the cause of the problem may be unclear. For example, the physician might suspect that a particular cardiac event has not been correctly identified by the cardiac stimulating device because the amplitude of a particular corresponding cardiac signal fell below the threshold level programmed into the device. However, an accurate determination of the magnitude of the cardiac signal in a specific portion of the cardiac waveform generally necessitates a careful measurement of the displayed signal, typically using calipers or some other mechanical measurement instrument. Physical measurement of the displayed cardiac signal on the display monitor or chart recorder output is slow, may be cumbersome, and may introduce undesirable measurement errors.
In an effort to alleviate some of the difficulties associated with physically measuring the cardiac data, systems of the type described in the aforementioned U.S. Pat. No. 4,791,936 to Snell et al. have been developed that display certain supplemental data, such as the time that has elapsed between R-waves or other cardiac events. However, frequently data is required that is related to an event that the cardiac stimulating device did not identify properly. If the cardiac stimulating device fails to identify the necessary cardiac event, such as an R-wave, then it is generally not possible to provide supplemental data related to that event. It would therefore be desirable for the physician to be able analyze specific portions of IEGM or EKG waveforms more rapidly and more completely, which would allow the physician to adjust the settings of a cardiac stimulating device more accurately.